Silica gels, whether hydrogels or aerogels, are well known. (See Iler, "The Chemistry of Silica", John Wiley & Sons 1979, ISBN 0-471-02404-X). Reference may also be had to U.S. Pat. No. 2,856,268 to L. O. Young and dated Oct. 14, 1958 for preparation of hydrogels suitable for the treatment described herein. Aerogels, or xerogels as they may be called, are dried hydrogels. They are also amorphous. Reference may also be had to the article by Mitchell "The Surface Properties of Amorphous Silicas" printed in Chemistry and Industry, June 4, 1966 at pages 924-932. These references are incorporated herein by reference thereto.
Experience has shown that almost any physical treatment performed on a silica gel after its formation results in a decrease in pore volume (PV) if we consider pore volume to be the difference between the volume of the gel and the volume of SiO.sub.2 in this gel. Silica gels as initialy formed may have a wide range of pore volumes from about 2 to about 50 cm.sup.3 /gm. depending on many factors as shown in the art. Those of particular commercial interest have pore volumes less than 10 cm.sup.3 /gram, e.g., about 4.5 cm.sup.3 /gm. Thus, washing to remove acid results in a decrease in pore volume. Aging results in a still further reduction in pore volume. By the time a silica gel with an initial pore volume of, e.g., 4.5 cm.sup.3 /g. is dried and milled to its final particle size in the range of 2 to 15 microns, its pore volume has decreased to from 20% to 39% of its original pore volume.
According to the present invention, we are able to stabilize the pore volume so that it decreases substantially less during physical treatment than that experienced with similar gels not treated in accordance with this invention. This is commercially significant in at least one area of use of the gels hereof; for example, the flatting of coating compositions, e.g, paints, varnishes, lacquers, enamels, and the like. The higher the pore volume, the lower the density of the gel particles. Hence, a smaller weight of silica particles of a predetermined size can be used effectively to produce a desired degree of flatting.